In high intensity discharge lamps, light is generated when an electric current is passed through a gaseous medium. The lamps have variable resistance characteristics that require operation in conjunction with a ballast to provide appropriate voltage and current limiting means. Control of the voltage, frequency and current supplied to the lamp is necessary for proper operation and determines the efficiency of the lamp. In particular it determines the size and weight of the required ballast.
The appropriate voltage, frequency and current for efficient running of a lamp in its normal operating state is not appropriate for the lamp during its warm-up stage. A high intensity discharge lamp typically takes several minutes to warm up from striking to its normal operating state. Initially the lamp is an open circuit. Short pulses of current are sufficient to strike the lamp provided they are of adequate voltage. Subsequent to striking, the lamp's resistance drops radically. The resistance then slowly rises during warm-up to its normal operating level. Hence, subsequent to striking and during warm-up the current of the lamp must be limited to prevent internal lamp damage.
A loss of power causes the lamp to extinguish. After a suitable cooling period the striking and warm-up phase must be repeated. The lamp's ballast system must detect and respond effectively and efficiently to the situation.
At times during warm-up high intensity discharge lamps exhibit "bulb rectification." For reasons not completely clear the lamp temporarily conducts in only one direction. A ballast system must achieve its objectives while accommodating this situation.
The prior art, as represented by U.S. Pat. Nos. 4,240,009, Paul, and 4,415,839, Lesea, regulate the current in the ballast system during the warm-up phase of a high intensity discharge lamp based on monitoring current and/or wattage consumption, or on monitoring power consumption alone. The prior art does not teach current regulation during warm-up based on monitoring the radiant energy, brightness, or the heat generated by the lamp itself.
Moreover, the prior art has not been able to produce a commercial feasible high power solid state ballast system for operation in, for example, high wattage mercury vapor lamps, that combines such features as low cost, light weight and inexpensive parts with efficiency and long life. The present invention overcomes these prior design limitations and presents a commercially feasible high power ballast (high wattage mercury vapor is the preferred embodiment) using precision control of current through relatively low power switches The present invention combines simplicity of design, light weight, small size and inexpensive parts with high efficiency and a probable longer bulb and ballast life due to the method of the controlled low current start up.
Therefore, it is a feature of the present invention to provide a self-adjusting ballast system for high intensity discharge lamps in an improved manner wherein precision control of the start up characteristics of the lamp is provided by means sensitive to the radiant energy or heat of the lamp.
It is another feature of the present invention to provide a self-adjusting ballast system for high intensity discharge lamps in an improved manner wherein, current imbalance in the alternating current lamp circuit is controlled by means of a current sensor in series with at least one switch.
It is another feature of the invention to provide a self-adjusting ballast system for high intensity discharge lamps in an improved manner wherein the system immediately resets itself to the initial strike state if the lamp extinguishes.